1. Field of the Invention
The present invention relates to a gray cast iron alloy for a friction element of a friction clutch and to a friction element formed from such an alloy.
2. Description of the Related Art
The demand for higher maximum transmittable torques in friction clutches used in modern motor vehicles is increasing because the drive units currently in use are capable of delivering very high torques. These comparatively high transmitted torques represent a considerable load on the various structural component parts of a friction clutch, particularly the cast structural component parts thereof. However, modern vehicles also require that the installation space for a friction clutch of this type be kept as small as possible, particularly also because torsional vibration dampers in the form of dual-mass flywheels or the like are often used in powertrains. As a result of the resulting smaller or weaker dimensioning of different cast structural component parts such as, for example, flywheels, problems such as breakage and cracks may occur due to the high thermal loading in friction operation.
Iron alloys that are used for producing flywheels or pressure plates in friction clutches include, for example, spheroidal graphite (SGI) alloys and vermicular graphite (CGI) alloys. These two types of cast iron alloys are relatively costly to produce and, moreover, are difficult to work with. Further, these materials have a high modulus of elasticity and a comparatively low thermal conductivity so that, as a result, a strong screen behavior is generated particularly by the heat occurring in friction operation. The uneven loading of the friction facings of the clutch disk which occurs due to the characteristics of these alloys generates uneven and excessive wear on the different structural parts which rub against one another and reduces the maximum torque which can be transmitted.
It is an object of the present invention to provide an alloy for a friction element of a friction clutch and a friction element produced from the same which have improved operating characteristics and that are simple to produce.
The object is met according to an embodiment of the present invention by a gray cast iron alloy for a friction element of a friction clutch having a friction surface arranged for frictional contact with a clutch disk, wherein the alloy contains:
3.0-3.4 percent by weight C;
1.8-2.3 percent by weight Si;
0.4-0.8 percent by weight Mn;
0.0-0.35 percent by weight P;
0.0-0.125 percent by weight S;
0.4-0.6 percent by weight Mo; and
a remainder comprising iron and production-related impurities and/or additives.
The use of a gray cast iron alloy (GCI), i.e., a cast iron alloy containing flake graphite, for a friction element used in a friction clutch produces a structural component part with appreciably higher thermal stability than the alloys used in the prior art. In addition, the introduction of the alloy constituent molybdenum increases the strength of the structural component part. In particular, the above-indicated weight-percent proportions of the different alloy constituents, carbon, silicon, manganese, phosphorus, sulfur and molybdenum, produces a structural component part which satisfies in an outstanding manner the requirements for such a structural component part in driving operation.
According to a further embodiment, the present invention provides a friction element for a friction clutch with a friction surface for frictional contact with a clutch disk, wherein the friction element is formed of the above-described alloy or which is formed of flake graphite.
The friction element may, for example, be a pressure plate of a friction clutch, a flywheel mass part of a friction clutch, a flywheel or part of a multi-mass flywheel, or an intermediate plate of a multidisk clutch. The multi-disk clutch may comprise a clutch disclosed in U.S. Pat. No. 6,276,505, the entire contents of which are enclosed herein by reference. To adapt a friction element produced from the alloy according to the invention to the respective requirements in an improved manner, the friction element may be subjected to stress-relief annealing at 450xc2x0 C. to 600xc2x0 C., preferably 500xc2x0 C. to 550xc2x0 C., for a period of at least 2.5 hours, preferably at least 3 hours after the friction element is cast in a casting process.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims. It should be further understood that the drawings are not necessarily drawn to scale and that, unless otherwise indicated, they are merely intended to conceptually illustrate the structures and procedures described herein.